The same bacteria that commonly cause relatively innocuous cases of strep throat in school-age children and young adults appears to be evolving into a menacing, more virulent form. In recent years, it has produced a severe and potentially lethal--though rare--infection, most often in adults. The ubiquitous strain of bacteria known as Group A streptococcus causes the pneumonia that swiftly killed Muppeteer Jim Henson in May.

The same bacteria that commonly cause relatively innocuous cases of strep throat in school-age children and young adults appears to be evolving into a menacing, more virulent form. In recent years, it has produced a severe and potentially lethal--though rare--infection, most often in adults. The ubiquitous strain of bacteria known as Group A streptococcus causes the pneumonia that swiftly killed Muppeteer Jim Henson in May.